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AIRCRAFT GAS TURBINE ENGINES
Subcourse No. AL0993
EDITION 5
US Army Aviation Logistics SchoolFort Eustis, Virginia
Nineteen Credit Hours
SUBCOURSE OVERVIEW
Fulfilling the Army's need for engines of simple design that are easy
to operate and maintain, the gas turbine engine is used in all
helicopters of Active Army and Reserve Components, and most of the
fixed-wing aircraft to include the Light Air Cushioned Vehicle (LACV).
We designed this subcourse to teach you theory and principles of the
gas turbine engine and some of the basic army aircraft gas turbine
engines used in our aircraft today.
There are no prerequisites for this subcourse.
This subcourse reflects the doctrine which was current at the time it
was prepared. In your own work situation, always refer to the latest
publications.
TERMINAL LEARNING OBJECTIVE
ACTION: You will describe the operation of major engine systems and
assemblies; describe the testing, inspection, and
maintenance of engine systems and assemblies; and
recognize various components.
CONDITION: Given information about the gas turbine engine, you
will work at your own pace in an environment of your
own choice, without supervision.
STANDARD: To demonstrate competency of this task, you must achieve a
minimum of 75% on the subcourse examination.
1 AL0993
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LESSON TITLE CREDIT HOURS
1 Theory and Principles of Gas Turbine Engines......... 2
2 Major Engine Sections................................ 2
3 Systems and Accessories.............................. 2
4 Testing, Inspection, Maintenance, and Storage
Procedures........................................... 2
5 Lycoming T53......................................... 2
6 Lycoming T55......................................... 27 Solar T62 Auxiliary Power Unit....................... 2
8 Allison T62, Pratt & Whitney T73 and T74,
and the General Electric T700........................ 2
Examination.................................................... 3
TOTAL 19
LESSON 1 ASSIGNMENT SHEET
LESSON 1.................Theory and Principles of Gas Turbine Engines.
CREDIT HOURS.............2.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 1.1-1.14.
MATERIALS REQUIRED.......None.
LESSON OBJECTIVE.........To enable you to describe the theory of a
gas turbine engine and its principles of
operation.
*** IMPORTANT NOTICE ***
THE PASSING SCORE FOR ALL ACCP MATERIAL IS NOW 70%.
PLEASE DISREGARD ALL REFERENCES TO THE 75% REQUIREMENT.
2 AL0993
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Weight True-False
(Answer A for true or B for false.)
3 1. In cold weather, gas turbine engines take a long time to
warm up to operating temperatures.
3 2. The Brayton cycle has the same four basic operations asthe Otto cycle, but it performs them simultaneously.
3 3. When air flows through a smaller section of a duct, it
increases in velocity and decreases in pressure and
temperature.
3 4. The turbojet aircraft is a high-speed, high-altitude
one.
3 5. The Army uses both turbojet and gas turbine engines.
Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.)
FIRST GROUP
Gas turbine engines have advantages and disadvantages.
Evaluate the following statements according to the
information in your text.
3 6. The power-to-weight ratio is 5.60 shp per pound for a
typical reciprocating engine but only .67 shp per pound
for a gas turbine engine.
3 7. The turbine engine has fewer moving parts than the
reciprocating engine.
3 8. Foreign object damage is a major problem for a gas
turbine engine.
3 9. A reciprocating engine uses less oil than a gas turbine.
3 10. They cost a great deal more than reciprocating engines.
3 11. They accelerate much faster than reciprocating engines.
3
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FOURTH GROUP
About turboprop and turboshaft engines, which of
questions 22 through 26 are true and which false?
3 22. In Army aircraft, rotational shaft power is produced bythe same turbine rotor that drives the compressor.
3 23. They do not eject high-velocity gases to obtain thrust.
3 24. A free-power turbine allows the power output shaft to
turn at a constant speed.
3 25. A free-power turbine is linked to the compressor turbine
mechanically.
3 26. The power producing capability is variable to take careof different loads on the power shaft.
FIFTH GROUP
Evaluate the following five questions on illustrating
the principle of jet propulsion by a toy balloon.
2 27. If it is inflated and the stem is sealed, the pressure
is equal on all internal surfaces.
2 28. If the stem is released, the balloon moves in adirection towards the open end.
2 29. The jet of air coming from the opened end of an inflated
balloon pushes against the outside air.
2 30. A convergent nozzle is created when the stem of the
balloon is released.
2 31. High internal pressure acting on the skin area opposite
the stem is what moves the balloon.
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Matching
In questions 32 through 37, match the statements in
column I with the laws or principles of physics in
column II by writing the proper letter by each question.
Each item in column II may be used once, more than once,or not at all.
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LESSON 2 ASSIGNMENT SHEET
LESSON 2.................Major Engine Sections.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 1.15-1.25.
LESSON OBJECTIVE.........To teach you to distinguish between the five
major sections of gas turbine engines.
CREDIT HOURS.............2
Weight True-False
(Answer A for true or B for false.)
3 1. Helicopter turboshaft engines develop thrust in their
exhaust ducts.
3 2. In an engine destination, a three-digit dash number
means that the engine was procured after the new Army
procurement system went into effect.
3 3. Engine idling is one of the two most severe operating
periods for combustion chambers.
3 4. An axial-centrifugal-flow compressor is made up of two
sets of axial-flow compressors and one centrifugal-flow
compressor.
3 5. When a number is placed after N, as is N2, it refers to
a specific system in the gas turbine engine.
7 AL0993
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Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.)
FIRST GROUP
About the three basic compressors used in gas turbine
engines, your text tells you that:
3 6. An axial-flow compressor is less susceptible to foreign-
object damage than a centrifugal-flow one.
3 7. Dual compressors are mounted on the same shaft and turn
at the same speed.
3 8. The centrifugal impeller in the axial-centrifugal-flowcompressor decreases the air velocity.
3 9. A centrifugal-flow compressor is made up of two rotors
and a compressor manifold.
3 10. A series of alternating rotor and stator vane stages
makes up the axial-flow compressor.
SECOND GROUP
The turbine section of a gas turbine engine transformsenergy into shaft horsepower. Which of these five
statements are true and which false about the different
types of turbines?
2 11. Axial-flow turbines are less expensive and easier to
manufacture than radial-flow turbines.
2 12. A single-rotor turbine has its power developed by one
rotor.
2 13. All gas-turbine-power aircraft in the Army today use theaxial-flow turbine.
2 14. Axial-flow turbines are invariably the single-rotor
type.
2 15. A single-rotor turbine is used where low weight and
compactness are necessary.
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THIRD GROUP
The next six questions about compressor construction
tell you that:
2 16. A centrifugal-flow compressor is made of either heat-treated forged aluminum or of cast aluminum.
2 17. All manufacturers use the split compressor case for
easier inspection.
2 18. Shrouds are used on stator vanes to provide an air seal
between rotating and stationary parts.
2 19. In an axial-flow compressor, blades fit tightly in the
turbine disk to reduce vibrational stress.
2 20. In both the axial- and centrifugal-flow compressors, the
fit between the compressor and its case is important.
2 21. In a centrifugal-flow compressor, the rotor may be
balanced by using balancing weights in the hub of the
compressor.
FOURTH GROUP
Evaluate the following five statements about engine
model designations by marking an "X" under A for true orunder B for false.
2 22. Each engine model designation begins with a letter or
two letters.
2 23. The letters TP at the beginning of the designation
identify a turboprop engine.
2 24. Even when a production model is changed, the dash number
remains the same.
2 25. The letter or set of letters following the assigned
model number identifies the manufacturer.
2 26. The Air Force always uses even numbers, both for the
assigned number and the dash number.
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FIFTH GROUP
Which of the following five questions are true and which
false about the compressor section of a gas turbine
engine?
2 27. Compressor efficiency determines the power necessary to
create the pressure for a given airflow and affects the
temperature in the combustion chamber.
2 28. The highest total air velocity is at the inlet of the
combustion section.
2 29. The highest static pressure is at the inlet of the
diffuser.
2 30. The volume of air pumped by the compressor isproportional to the rpm of the rotor.
2 31. The compressor is made up of alternating rotating and
stationary vane assemblies.
SIXTH GROUP
About turbine construction, your text tells you that:
2 32. A turbine rotor operates at high temperatures, at high
speeds.
2 33. A fir tree design is used in attaching the blades to the
disk.
2 34. A moment weight number is stamped on each rotor blade to
preserve rotor balance when they are replaced.
2 35. The fir tree blade design eliminates any need for rivets
or other locking devices.
2 36. Shrouding is used in turbines subject to the highesttemperatures and highest speeds.
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Matching
In questions 37 through 41, match the definitions in
column I with the terms or symbols they define in column
II by writing the proper letter by each question. Each
item in column II may be used once, more than once, ornot at all.
Multiple Choice
(Each question in this group contains one and only one
correct answer. Make your choices by circling the
proper letter for each question in the lesson book. )
2 42. Which of these is the most commonly used in Army
aircraft?
A. Annular reverse-flow combustion chamber.
B. Can-annular combustion chamber.
C. Can combustion chamber.
D. Annular straight-flow combustion chamber.
2 43. Which of these is not part of a combustion chamber?
A. Perforated inner liner.
B. Stator vanes.
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C. Casing.
D. Fuel nozzles.
2 44. Which statement is true of the air and fuel in a
combustor?
A. The only place for air to flow into the combustor is
through the first combustion air holes in the liner.
B. The combusted gases are exhausted directly to the
air.
C. A ratio of 15 parts of air to 1 part of fuel by
weight is the correct mixture.
D. Seventy-five percent of the compressor air is used
for burning.
2 45. Which of these is true of the annular combustion
chamber?
A. It contains individual combustion chambers.
B. It has an adapter through which compressor air
enters the individual chambers.
C. Crossover tubes connect all liners.
D. Combustion takes place in a space between the
combustor liner and the turbine shaft.
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LESSON 3 ASSIGNMENT SHEET
LESSON 3.................Systems and Accessories.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 2.1-2.26.
LESSON OBJECTIVE.........To enable you to recognize and describe the
gas turbine engine fuel and oil systems andtheir components.
CREDIT HOURS.............2
Weight True-False
(Answer A for true or B for false.)
2 1. The most common ignition system in Army gas turbineengines produces high tension voltage by conventional
induction coils.
2 2. Fuel is invariable conducted between the parts of the
system through flexible lines.
2 3. Oil is cooled in some gas turbine engines by
transferring the heat in the oil to the fuel flowing to
the fuel nozzles.
2 4. Because the burning process is continuous in a gasturbine engine, the amount of cooling air is greater
than the amount of combustion air.
2 5. The pressurizing and drain dump valves may be used to
prime the fuel control.
2 6. The fuel pump may be built into the fuel control, or it
may be a separate component.
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Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.)
FIRST GROUP
Gas turbine engines may have several fuel filters.
Which of the following statements about them are true
and which false?
2 7. Usually a filter includes a relief valve to open at a
specified differential pressure.
2 8. A filter made of stainless steel mesh cloth used to
filter particles larger than 40 microns is called a
cylindrical screen filter.
2 9. Filters may be located in other places in the fuel
system besides the main lines.
2 10. A paper cartridge filter removes particles larger than
100 microns.
2 11. A cylindrical screen filter is used where the fuel
pressure is high.
SECOND GROUP
The text tells you that ignition systems are of three
general types. Which of these questions on them are
true and which false?
2 12. All gas turbine engines have two or more igniter plugs.
2 13. The high-energy capacitor type of ignition system
actually produces a small amount of energy.
2 14. The annular-gap igniter plug electrode can operate at acooler temperature than that of the constrained-gap
plug.
2 15. In gas turbine engines, ignition takes place in
microseconds.
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2 16. Electrodes of gas turbine igniters and of conventional
spark plugs can accommodate the same amount of energy.
2 17. Severely damaged ignition exciter units should be
handled with forceps or gloves because they may be
radioactive.
THIRD GROUP
About lubrication systems for gas turbine engines, your
text tells you that:
2 18. Oil for gas turbine engines has a conventional petroleum
base.
2 19. In most turbine engines, the oil is stored separately
from the engine.
2 20. Pressure developed by a gear pump has no relation to
engine rpm.
2 21. Oil drawn from the engine by scavenge pumps is
discarded.
2 22. A gerotor pump has a tooth missing in its inner toothed
element.
2 23. The oil tank is usually made of welded aluminum orsteel.
2 24. The sole purpose of the lubrication system is to reduce
friction.
FOURTH GROUP
Which of the following six statements about the
automatic and manual fuel control systems are true and
which false?
2 25. The amount of fuel needed to run the engine varies with
inlet air temperature and pressure.
2 26. Automatic fuel control is provided by the speed
governor.
2 27. The manual throttle control compensates for altitude and
temperature.
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2 28. Most fuel controls in use today are hydromechanical.
2 29. The speed governor is merely a spring attached to the
manual metering valve.
FIFTH GROUP
About the starting-fuel system, your text tells you
that:
2 30. All gas turbine engines have the same number of start-
fuel nozzles.
2 31. The pilot turns on the start-fuel solenoid switch in the
cockpit.
2 32. The start fuel flows directly from the external line tothe fuel nozzles.
2 33. The nozzles spray atomized fuel in the combustion
chamber during starting.
2 34. The start fuel system is shut off when the engine is
running on the main fuel system.
SIXTH GROUP
On the subject of fuel nozzles, your text says that:
2 35. A simplex nozzle can provide as satisfactory a spray
pattern as a duplex nozzle.
2 36. All gas turbine engines use fuel nozzles.
2 37. A duplex nozzle must have a fuel-flow divider.
2 38. A fuel-flow divider separates the fuel into low and high
pressure supplies.
2 39. A spring-loaded valve is set to open at a specific fuel
pressure; this is the fuel divider.
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SEVENTH GROUP
Your text discusses the hydromechanical type of fuel
control. Evaluate the next five statements on the
subject.
2 40. Fuel flow has no relation to exhaust gas temperature.
2 41. Fuel control may be achieved by varying the orifice of
the metering valve.
2 42. As the engine accelerates and airflow through the engine
increases, fuel flow is decreased.
2 43. A fuel control with a longer acceleration time than
would be used for a reciprocating engine must be used
because engine compressors are subject to surges andstalls.
2 44. One of the factors that limits engine operation is
temperature of the compressor inlet.
Matching
In questions 45 through 50, match the statement in
column I with the instruments to which they apply in
column II by writing the proper letter by each question.
Each item in column II may be used once, more than once,or not at all.
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Column I
2 49. Is a sensitive milli-
voltmeter.
2 50. Registers engine androtor rpm for rotary-
wing aircraft.
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LESSON 4 ASSIGNMENT SHEET
LESSON 4.................Testing, Inspection, Maintenance, and
Storage Procedures.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 3.1-3.15.
LESSON OBJECTIVE.........To enable you to describe the procedures fortesting, inspection, maintenance, and
storage of aircraft gas turbine engines.
CREDIT HOURS.............2
Weight True-False
(Answer A for true or B for false.)
2 1. To find out what kind of metal is in used oil, the wave
length of the light from burning it is measured.
2 2. Troubleshooting charts to analyze, isolate, and correct
engine malfunctions are found in the engine TM.
2 3. In selecting a method for cleaning an engine, make sure
that anodizing or dichromating is not removed from the
surface.
2 4. If an exhaust gas temperature system needstroubleshooting, a jetcal analyzer is used.
2 5. The symbol "D" on a maintenance allocation chart means
direct support maintenance.
2 6. Special instructions are required at specific intervals
between scheduled inspections.
2 7. Maintenance and inspection of gas turbine engines are
appreciably more difficult than those of reciprocating
engines.
19 AL0993
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2 17. No corrections of engine problems are made during
testing.
2 18. An engine may also be tested by a mobile engine test
unit.
2 19. If an engine fails a test run in a test cell, not only
is it disassembled and checked for faults, but many
previous engines are also.
2 20. Engine information obtained during testing does not
become a part of the engine record.
THIRD GROUP
About cleaning compressor rotor blades, the reference
text tells you that:
3 21. The temperature-sensing element on the T53 engine is
cleaned with a spray of clean, fresh water.
3 22. A steady increase in EGT during normal operation is one
indication that the compressor rotor blades may need
cleaning.
3 23. If the engine has been operating in salt-air areas,
spraying it with fresh water is all the cleaning
necessary.
3 24. Always refer to the TM for the specific engine to find
the exact procedure for cleaning it.
3 25. The T53 engine does not have to be run after it is
cleaned.
3 26. The engine TM specifies a definite performance point at
or below which the compressor blades must be cleaned.
FOURTH GROUP
These statements are on gas turbine engine overhaul andrepair, storage, and preservation. Evaluate them by
marking them true or false.
3 27. An engine does not go into the flyable storage category
until it has not been operated for six days.
3 28. After the TBO for an engine is established, it is not
changed.
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3 29. No special tools are needed to disassemble an engine.
3 30. Permanent storage is a depot-level function.
3 31. When disassembling or assembling an engine, instructions
in the TM must be followed precisely.
3 32. The degree of preservation of an engine is governed by
the length of time it is expected to be in storage.
FIFTH GROUP
Which of the following statements agree with the
information on engine vibrations given in your text?
2 33. Forced vibrations are invariably caused by improper
assembly of the components.
2 34. In a gas turbine engine, imbalance of rotating parts is
the main cause of vibrations.
2 35. A vibration transducer is used to analyze the force
generated by the amount of imbalance and the rotating
speed.
2 36. Imbalance is measured in mils.
2 37. The Engine Vibration Test Data Sheet gives the figuresfor maximum permissible engine vibration.
2 38. Externally excited vibrations may also be caused by
imbalance of rotating engine components.
Matching
In questions 39 through 44, match the statements in
column I with the cleaning method to which they apply in
column II by writing the proper letter by each question.
Each item in column II may be used once, more than once,or not at all.
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LESSON 5 ASSIGNMENT SHEET
LESSON 5.................Lycoming T53.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 4.1-4.23.
LESSON OBJECTIVE.........To enable you to describe the operation of
the T53 engine and its sections, its modelsand specifications, and its major engine
systems and assemblies.
CREDIT HOURS.............2
Weight True-False
(Answer A for true or B for false.)
2 1. The T53 gas turbine engine includes an annular-flow path
for the air or hot gases.
2 2. The function of the diffuser is to increase air pressure
in the area.
2 3. The electric torquemeter is used on the T53-L-13 engine.
2 4. The T53-L-701 has a single-stage power turbine.
2 5. Besides cooling internal engine components, the internalcooling system pressurizes the No. 1, 2, and 3 bearing
seals.
2 6. The right side of the engine is determined by viewing
the engine from the front.
24 AL0993
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2 7. Varying the angle of the inlet guide vanes changes the
N1 compressor speed.
2 8. The reduction gearassembly in the T53-L-701 is smaller
than that in the T53-L-13.
Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.)
FIRST GROUP
In the discussion on the inlet housing assembly, your
text tells you that:
3 9. Split power gearing allows greater horsepower.
3 10. Its sole purpose is to contain the components inclosed
within it.
3 11. The aft side of the No. 1 main bearing is completely
sealed by a radial labyrinth seal.
3 12. In the T53-L-701, power is transmitted to the propeller
shaft by both the primary and secondary drive systems.
3 13. In the T53-L-13 engine, the sun gear drives the output
gearshaft directly.
SECOND GROUP
Airflow through the engine shows how a gas turbine
engine works. Which of the following statements about
it are true and which false?
2 14. When direction of airflow is reversed in the combustion
area, air velocity and pressure increase.
2 15. Vanes in the diffuser air passageway direct the air into
the compressor section.
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2 16. In the combustion area, the air is used solely to aid
combustion.
2 17. After flowing through the two-stage power system, the
gas passes through the exhaust diffuser passageway into
the atmosphere.
2 18. When air enters the combustion area, its flow direction
is reversed.
THIRD GROUP
Which of these is true of an accessory drive assembly?
2 19. The N1 accessory drive gearbox assembly is mounted on
the upper left side of the gearbox housing.
2 20. Both the N1 and N2 assemblies receive their drive from
the same kind of gear.
2 21. The N1 has drive pads for the fuel control, the starter
generator, and the gas producer tachometer generator.
2 22. The N2 overspeed governor and tachometer drive assembly
is on the underside of the engine inlet housing.
2 23. The fuel control overspeed governor is driven by the N2
assembly.
FOURTH GROUP
Evaluate the following statements about the torquemeter
used on the L-13 model by marking them true or false.
3 24. Because it uses engine oil, it is part of the
lubrication system.
3 25. Two circular plates make up the mechanical portion of
the torquemeter.
3 26. The stationary plate is attached to the reduction gear
assembly.
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3 27. Air pressure does not affect torque indications in this
particular torquemeter because the transmitter cancels
the air pressure effect.
3 28. The stationary and movable plates are separated by steel
balls.
FIFTH GROUP
Your text tells you which of the following about the
operation of gas turbine engines?
2 29. More of the gas energy from the combustion chamber is
used by the power turbines than by the gas producer
turbine.
2 30. When N1 speed reaches 8 to 13 percent, main fuel flowsinto the combustion chamber.
2 31. Combustion gases pass through the gas producer nozzle
assemblies and next go to the blades of the power
turbine rotor assemblies.
2 32. The power turbine rotor assemblies are connected to the
power shaft.
2 33. The burning starting fuel ignites the main fuel in the
combustion chamber after the fuel regulator valve opens.
SIXTH GROUP
The compressor assembly is made up of five axial
compressor rotor disks and one centrifugal impeller,
with their housings. Evaluate the following statements
about them.
2 34. Roll pins and lock plates secure the compressor blades
in dovetail slots in the rotor disks.
2 35. Stators direct airflow to the following sets of rotating
compressor blades; the fifth stator assembly has a row
of exit guide vanes which direct airflow to the
compressor impeller.
2 36. Only one half of the compressor housing may be removed
at one time because the housing is used for structural
support.
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2 37. Stainless steel inserts are put between some stator vane
rows to increase air velocity.
2 38. The powershaft is attached inside the compressor rotor
sleeve and rotates with it.
2 39. Compressor bleed air flows through passages in the axial
compressor housing.
SEVENTH GROUP
About the combustor assembly on the T53, your text tells
you that:
2 40. It is located forward of the diffuser housing assembly.
2 41. The turbine assembly includes the N1 assembly and the N2assembly.
2 42. The combustion chamber drain valve remains open
throughout engine operation and closes at engine
shutdown.
2 43. No cooling air is needed in the exhaust diffuser.
2 44. The combustion chamber housing is annular and is made of
steel.
2 45. The average temperature of the gas stream is measured in
the turbine inlet area.
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LESSON 6 ASSIGNMENT SHEET
LESSON 6.................Lycoming T55.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 5.1-5.22.
LESSON OBJECTIVE.........To enable you to recognize and describe the
Lycoming T55 gas turbine engine.
CREDIT HOURS.............2
Weight True-False
(Answer A for true or B for false.)
3 1. The power turbine extracts velocity energy from the hot
gases and transmits mechanical power to the outputshaft.
3 2. In figure 5.5, station No. 4 is located from the
beginning of the centrifugal compressor to the air
diffuser.
3 3. A 3.75-gallon oil tank is contained in the inner housing
of the inlet housing assembly.
3 4. The L-11 model has a two-stage gas producing turbine.
3 5. The right and left sides of the engine are determined by
looking at the engine from the front.
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Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.)
FIRST GROUP
The T55-L-l1 engine fuel system includes:
3 6. Fourteen vaporizing tubes instead of 28 dual atomizing
nozzles.
3 7. A fuel control unit is made up of the flow control and
computer sections.
3 8. Actuation of the compressor bleed band by the fuelcontrol section.
3 9. A cooler that uses fuel to cool engine oil.
3 10. Main and start fuel manifolds at the rear of the
combustion chamber assembly.
SECOND GROUP
Which of the following statements about the sections of
the T55-L-1l are true and which false?
3 11. The compressor has a seven-stage axial compressor.
3 12. The divergent shape of the diffuser decreases air
pressure and increases velocity.
3 13. The struts between the inner and outer air inlet housing
are hollow, with passages for oil and accessory drive
shafts.
3 14. The combustor has 14 fuel nozzles in each one of its twomain fuel manifolds.
3 15. A variable inlet guide vane assembly is mounted in the
front of the compressor housing.
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Weight
THIRD GROUP
The following statements are about various systems in
the T55-L-l1 engine. Evaluate them by marking them true
or false.
2 16. The anti-icing system for the variable inlet guide vanes
uses hot scavenge oil.
2 17. Bearing seals are pressurized by some of the cooling
air.
2 18. The purpose of an interstage air bleed system is to
avoid compressor stalls and to increase compressor rotor
acceleration.
2 19. The control system for the variable inlet guide vanesschedules their positions according to gas producer
speed and compressor inlet temperature.
2 20. Several passages throughout the engine receive air from
the main airflow to cool components.
2 21. The torquemeter system gives a reading in the cockpit of
percent of torque.
FOURTH GROUP
The lubrication system in the T55 engine has the
following characteristics.
2 22. All oil filters can be changed at intermediate level
maintenance.
2 23. The low-level warning switch in the cockpit signals when
a 2-hour supply of usable oil remains.
2 24. After oil leaves the main oil pump, it goes through a
filter in the accessory gearbox.
2 25. Chip detectors activate caution lights in the cockpit.
2 26. The main oil pump is used entirely to maintain pressure
in the oil system.
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Weight
2 27. The entire lubrication system is contained in the
engine.
FIFTH GROUP
Which of the following statements describe the T55 gasturbine engine?
2 28. The accessory drive section is part of the annular flow
path.
2 29. The speed of the power output shaft is the same as that
of the power turbine.
2 30. The airflow is reversed twice in the combustor section.
2 31. In the combustor, the curl assembly reverses the airflowdirection for the first time.
2 32. The swirl cups in the combustor contain dual-orifice,
fuel-atomizing nozzles.
2 33. The T55 engine has six sections.
SIXTH GROUP
In comparing the different models of the T55, you find
that:
2 34. The normal shp for the L-l11 is the same as the military
shp for the L-7C.
2 35. The L-7's all have 28 dual-orifice fuel spray nozzles.
2 36. The L-7B has the most accurate electric torquemeter.
2 37. The L-1l model has a two-stage GP turbine.
2 38. All models of the T55 have the same shaft horsepower.
2 39. The CH-47C cannot use the T55-L-ll engine.
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Weight
Matching
Match the specifications in column I with the model to
which they apply in column II by writing the proper
letter by each question. Each item in column II may be
used once, more than once, or not at all.
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LESSON 7 ASSIGNMENT SHEET
LESSON 7.................Solar T62 Auxiliary Power Unit.
TEXT ASSIGNMENT..........Reference Text AL0993, paragraphs 6.1-6.12.
LESSON OBJECTIVE.........To enable you to describe the T62 APU, how
it operates, and its various components.
CREDIT HOURS.............2
Weight True-False
(Answer A for true or B for false.)
3 1. An hour meter attached to the engine records the
operating time of the engine.
3 2. In the turbine assembly, the shaft is supported by a
forward ball bearing and an aft roller bearing.
3 3. The OVSP light on the instrument panel goes on when the
horsepower reading reaches 70 percent.
3 4. Both models of the T62 invariably burn JP-4 gasoline.
3 5. Lubrication system pressure is maintained at 15 to 25
psi by a pressure relief valve.
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Weight
Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.)
FIRST GROUP
The T62 electrical system supplies power for the
ignition and engine electrical accessories. Evaluate
the following statements by marking them true or false.
3 6. The spark plug used for ignition operates all the time
the engine is running.
3 7. The speed switch is actually two switches in one
housing.
3 8. The ignition exciter converts input current to an
intermittent high-energy current.
3 9. All current in the APU is dc.
3 10. A switch shuts the engine down if the exhaust is too
hot.
3 11. The tachometer generator and the speed switch are
mounted on the accessory drive assembly.
SECOND GROUP
In a description of the T62 APU, you find the following
information.
3 12. The T62 APU is an item of ground support equipment.
3 13. It has its own hydraulic starter motor.
3 14. Each model of the T62 has different temperature limits.
3 15. The T62T-2A has a higher input speed than the T62T-16A.
3 16. It develops approximately 70 shaft horsepower.
3 17. Its compressor and turbine rotor are mounted back-to-
back on a single shaft.
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Weight
THIRD GROUP
Which of the following information about the T62 fuel
system is correct?
3 18. The flyweight assembly in the governor housing allows asmall amount of fuel to flow at all times.
3 19. For combustion, atomized fuel from the start fuel nozzle
is ignited by the spark plug.
3 20. As compressor pressure increases, so does fuel flow to
the main fuel injectors.
3 21. Fuel flow to the fuel injectors is not affected by
ambient air pressure.
3 22. When the fuel pressure switch opens, it actuates the
mechanism to start the engine.
FOURTH GROUP
About the operation of the T62, the reference text tells
you that:
2 23. During starting, when the hydraulic starter rotates the
compressor, air is drawn into the engine inlet.
2 24. The start fuel solenoid valve is opened when the speed
of the APU reaches 75%.
2 25. Power for the reduction drive assembly comes directly
from the compressor.
2 26. Fuel from the start fuel nozzle is ignited by a spark
plug.
2 27. Fuel goes into the combustor through six vaporizer
tubes.
FIFTH GROUP
If any of the APU operating limitations are exceeded,
protective devices shut the APU down.
2 28. An overspeed switch is set at 110 percent.
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Weight
2 29. When the instrument panel "Low Oil Press" light comes
on, the pilot must switch off the APU immediately.
2 30. Oil pressure must be more than 6 psi.
2 31. If the APU shuts off, the cockpit control switch must bemoved to the STOP position before restarting the engine.
2 32. The overspeed switch shuts off the engine fuel flow.
2 33. The pilot shuts off the APU when the instrument panel
OVSP light turns on.
Matching
Match the statement in column I with the assembly to
which it applies in column II by writing the properletter by each question. Each item in column II may be
used once, more than once, or not at all.
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Weight
3 5. The T63 TOT thermocouple assembly is a harness with four
probes.
3 6. The exhaust duct is bolted to the free-turbine case in
the T73 engine.
Cluster True-False
(Each of the following groups of questions is related to
the statement that precedes them. Write by each
question A for true or B for false.
FIRST GROUP
Evaluate the following statements about the T63 systems
by marking them true or false.
3 7. During start, acceleration, and stabilization at ground
idle rpm, fuel flow is metered entirely by the fuel
control (N1).
3 8. The lubrication system uses an oil mist on compressor,
gas producer turbine, and power turbine rotor bearings.
3 9. The gas producer fuel control and the power turbine
governor are not connected to each other.
3 10. The air bleed system is controlled by a valve whichbegins to close when a specific pressure ratio is
reached.
3 11. Torsional vibrations in helicopter rotor systems are
dampened by the check valve assembly and accumulator.
3 12. In the lubrication system, a check valve in the oil
filter outlet passage keeps oil in the tank from
draining into the engine.
3 13. If one of the two fuel pumps fails, the engine shutsdown.
SECOND GROUP
Among the details on the T74 engine, you find that:
3 14. The glow-plug ignition system is for quick starts at low
ambient temperature.
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Weight
3 15. Both pumps in the scavenge oil system are externally
mounted.
3 16. The heat exchanger uses heat from the oil lubricating
system to preheat the fuel in the engine fuel system.
3 17. The oil tank must be drained before changing the oil
filter.
3 18. One of the T74's turbines drives a compressor in the gas
generator section, and the other drives a reduction
gearing.
3 19. The reduction gearbox is at the front of the engine, and
the accessory gearbox is at the rear of the engine.
3 20. The compressor turns in a clockwise direction and thepower turbine shaft turns in a counterclockwise
direction.
3 21. The fuel-air mixture in the combustion chamber liner is
ignited by fourteen simplex nozzles.
THIRD GROUP
Basic to your knowledge of gas turbine engines should be
enough information about the T73 systems to evaluate the
following statements.
2 22. The ignition system constitutes the entire engine
electrical system.
2 23. Scavenge oil from the main bearings and gearbox empties
into a common tube that returns it to the tank.
2 24. Fuel goes from the fuel pump directly to the right and
left fuel manifolds.
2 25. An external tube on the left side of the engine carrieshot anti-icing air forward to the compressor.
2 26. All pressure oil lines in the lubrication system are
internal.
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Weight
FOURTH GROUP
About the two T63 engines, the text tells you that:
3 27. The lubrication system has one magnetic chip detector
plug in the accessory gearbox sump and one in thescavenge oil pressure line.
3 28. The gas producer and power turbine gear trains are both
contained in the accessory gearbox section.
3 29. The T63-A-5A is the one used on the OH-58 helicopter.
3 30. Both the power turbine and the gas producer turbine
rotate at the same speed.
3 31. The T63 compressor must not be cleaned with ordinarycleaning solvents, because they would dissolve the
plastic coating on the inside.
FIFTH GROUP
In the T73 engine:
2 32. The main pumping element raises the fuel pressure by
approximately 20 psi.
2 33. The gas producer rotor and the power turbine rotor turnin opposite directions.
2 34. Stages 5 through 9 of the compressor rotor shroud and
vane assembly are housed in the diffuser case.
2 35. The accessory drive gear is in the gas producer turbine
rotor assembly.
2 36. One duplex fuel nozzle matches each of the combustion
chambers.
2 37. High-pressure air for anti-icing and fuel heating is
bled off from the compressor inlet case.
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INTRODUCTION
Aircraft designers have always been limited by the efficiency of
the available powerplants. Their constant plea has been for higher
power, less weight, lower frontal area, better cooling
characteristics, and lower fuel consumption. These requirements have
been met to a certain degree by the designers of reciprocating
engines, but the design of the piston engine has been carried to sucha point that to obtain further increase in power, more cylinders
would have to be added. This would immediately raise more complex
problems, which must be solved before an increase in power can be
achieved.
The aircraft designers' pleas have been answered with the
development of the gas turbine engine. Since the end of World War
II, progress in the gas turbine field has been rapid. Development of
improved materials, high temperature metals, and better fuels should
expedite further progress in this held. The gas turbine engine's
greatest contribution to aviation is that it has lifted all previouslimits that were imposed by the reciprocating engine.
This text describes the operation, components, and systems of the
gas turbine engine. The first chapter includes an introduction to
gas turbine engines. Chapter 2 discusses the systems and accessories
such as fuel, oil, and electrical. Chapter 3 covers testing,
inspection, maintenance, and storage procedures. Chapters 4 through
10 describe in detail the gas turbine engines used in Army aircraft.
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Newton's Second Law of Motion. The second law states that an
imbalance of forces on a body produces or tends to produce an
acceleration in the direction of the greater force, and the
acceleration is directly proportional to the force and inversely
proportional to the mass of the body.
Newton's Third Law of Motion. The third law states that for
every action there is an equal and opposite reaction, and the two aredirected along the same straight line.
Bernoulli's Principle. This principle states that if the
velocity of a gas or liquid is increased its pressure will decrease.
The opposite is also true. If the velocity of a gas or liquid is
decreased its pressure will increase. This fact relates directly to
the law of conservation of energy.
Einstein's Law of Conservation of Energy. This law states
that the amount of energy in the universe remains constant. It is
not possible to create or destroy energy; however, it may betransformed.
Boyle's Law. This law states that if the temperature of a
confined gas is not changed, the pressure will increase in direct
relationship to a decrease in volume. The opposite is also true --
the pressure will decrease as the volume is increased. A simple
demonstration of how this works may be made with a toy balloon. If
you squeeze the balloon, its volume is reduced, and the pressure of
air inside the balloon is increased. If you squeeze hard enough, the
pressure will burst the balloon.
Charles' Law. This law states that if a gas under constant
pressure is so confined that it may expand, an increase in the
temperature will cause an increase in volume. If you hold the
inflated balloon over a stove, the increase in temperature will cause
the air to expand and, if the heat is sufficiently great, the balloon
will burst. Thus, the heat of combustion expands the air available
within the combustion chamber of a gas turbine engine.
Pressure and Velocity. Air is normally thought of in relation
to its temperature, pressure, and volume. Within a gas turbine
engine the air is put into motion so now another factor must beconsidered, velocity. Consider a constant airflow through a duct.
As long as the duct cross-sectional area remains unchanged, air will
continue to flow at the same rate (disregard frictional loss). If
the cross-sectional area of the duct should become smaller (convergent
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area), the airflow must increase velocity if it is to continue to
flow the same number of pounds per second of airflow (Bernoulli's
Principle). In order to obtain the necessary velocity energy to
accomplish this, the air must give up some pressure and temperature
energy (law of conservation of energy). The net result of flow
through this restriction would be a
decrease in pressure and temperature
and an increase in velocity. Theopposite would be true if air were to
flow from a smaller into a larger
duct (divergent area); velocity
would then decrease, and pressure
and temperature would increase. The
throat of an automobile carburetor is a
good example of the effect of airflow
through a restriction (venturi); even on the
hottest day the center portion of
the carburetor feels cool.
Convergent and divergent areasare used throughout a gas
turbine engine to control
pressure and velocity of the
air-gas stream as it flows
through the engine.
1.4. THEORY OF JET PROPULSION
The principle of jet
propulsion can be illustrated by
a toy balloon. When inflated andthe stem is sealed, the pressure
is exerted equally on all
internal surfaces. Since the
force of this internal pressure
is balanced there will be no
tendency for the balloon to move.
If the stem is released
the balloon will move in a
direction away from the escaping
jet of air. Although the flightof the balloon may appear
erratic, it is at all
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A source of power is now required to turn the compressor. To
extend the volume of air, fuel and ignition are introduced and
combustion takes place. This greatly expands the volume of gas
available.
In the path of the now rapidly expanding gases, another fan or
turbine can be placed. As the gases pass through the blades of the
turbine, they cause it to rotate at high speed. By connecting the
turbine to the compressor, we have a mechanical means to rotate the
compressor to replenish the air supply. The gases still possessing
energy are discharged to the atmosphere through a nozzle that
accelerates the gas stream. The reaction is thrust or movement of
the tube away from the escaping gas stream. We now have a simple
turbojet engine.
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The turbojet engine is a high-speed, high-altitude powerplant.
The Army, at present, has no requirement for this type of engine.
Because it is simple and easy to operate and maintain, however, the
Army does use the gas turbine engine. The simple turbojet engine has
primarily one rotating unit, the compressor/turbine assembly. The
turbine extracts from the gas stream the energy necessary to rotate
the compressor. This furnishes the pressurized air to maintain the
engine cycle. Burning the fuel-air mixture provides the stream ofhot expanding gas from which approximately 60 percent of the energy
is extracted to maintain the engine cycle. Of the total energy
development, approximately 40 percent is available to develop useful
thrust directly.
If we had ten automobile engines that would equal the total
shaft horsepower of a turbine engine, it would take six of these
engines to turn the compressor, and the other four would supply the
power to propel the aircraft. The amount of energy required to
rotate the compressor may at first seem too large; however, it should
be remembered that the compressor is accelerating a heavy mass(weight) of air towards the rear of the engine. In order to produce
the gas stream, it was necessary to deliver compressed air by a
mechanical means to a burner zone. The compressor, being the first
rotating unit, is referred to as the N1 system.
With a requirement for an engine that delivers rotational
shaft power, the next step is to harness the remaining gas stream
energy with another turbine (free turbine). By connecting the
turbine to a shaft, rotational power can be delivered to drive an
aircraft propeller, a helicopter rotor system, a generator, a tank,
an air cushion vehicle (ACV), or whatever is needed. The power shaftcan extend from the front, back, or from an external gearbox. All of
these locations are in use on various types of Army engines at
present.
The following sketch shows a turboshaft engine with the power
shaft extended out the front. The bottom sketch shows the same
engine with the power shaft extending out the back.
The basic portion of the turbine engine, the gas producer,
extracts approximately 60 percent of the gas stream energy
(temperature/pressure) to sustain the engine cycle. To developrotational shaft power, the remaining gas stream energy must drive
another turbine. In Army engines today, a power turbine that is free
and independent of the gas producer system accomplishes this task.
The power turbine and shaft (N2 system) are not mechanically connected
to the gas producer (N1 system). It is a free turbine. The
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gas stream passing across the turbines is the only link between these
two systems. The free-turbine engine can operate over wide power
ranges with a constant output-shaft speed.
In operation, the gas producer (N1) system automatically
varies its speed, thereby controlling the intensity of the gas stream
in relation to the load applied to the power (N2) shaft. This is
accomplished by a fuel metering system that senses engine
requirements. The free turbine design has revolutionized the methods
of application of shaft turbine engines. Why a shaft turbine? Why
is a perfectly good jet engine used to drive a propeller? Because in
the speed range that Army aircraft operate, the propeller or
helicopter rotor is more efficient. With a turbojet engine, power
(thrust) produced is roughly the difference between the velocity ofthe air entering the engine and the velocity of the air exiting from
the engine. Efficiency of the engine (power producer versus fuel
consumed) increases with speed until it is 100 percent efficient when
the forward
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speed of the engine is equal to the rearward speed of the jet. It is
this low efficiency at takeoff and at low cruising speed (i.e., 400
mph) that makes the turbojet engine unsuitable for use in Army
aircraft. The propeller does not lack efficiency at low speed; the
reverse is true, in that efficiency falls off at high speed. The
result is to harness the jet engine's gas stream energy to drive a
propeller or helicopter rotor system, thereby taking advantage of the
best features of both.
Aircraft reciprocating engines operate on the four-stroke,
five-event principle. Four strokes of the piston, two up and two
down, are required to provide one power impulse to the crankshaft.
Five events take place during these four strokes: the intake,
compression, ignition, power, and exhaust events. These events must
take place in the cylinder in the sequence given for the engine to
operate.
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horsepower (shp), which is used to drive the output shaft of the
engine. The gas then exits the engine through the exhaust section to
the atmosphere. Army helicopters use a divergent duct to eliminate
the remaining thrust. The various kinds of exhaust ducting are
discussed in detail with the engine using that particular ducting.
Figure 1.2. Typical Free-Power Turboshaft Engine.
1.10. TURBOJET
The turbojet is the engine in most common use today in high-
speed, high-altitude aircraft, not in Army aircraft. With this
engine, air is drawn in by a compressor which raises internal
pressures many times over atmospheric pressure. The compressed air
then passes into a combustion chamber where it is mixed with fuel to
be ignited and burned. Burning the fuel-air mixture expands the gas,
which is accelerated out the rear as a high-velocity jet-stream. In
the turbine section of the engine, the hot expanded gas rotates aturbine wheel which furnishes power to keep the compressor going.
The gas turbine engine operates on the principle of intake,
compression, power, and exhaust, but unlike the reciprocating engine,
these events are continuous. Approximately two-thirds of the total
energy developed within the combustion chamber is absorbed by the
turbine
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wheel to sustain operation of the compressor. The remaining energy
is discharged from the rear of the engine as a high velocity jet, the
reaction to which is thrust or forward movement of the engine. The
turbojet is shown schematically in figure 1.3.
Figure 1.3. Axial-Flow Turbojet Engine.
1.11. TURBOPROP ENGINE AND TURBOSHAFT ENGINE
The turboprop engine and turboshaft engines, shown in figures
1.4 and 1.5, are of the same basic type as the turbojet. Instead ofejecting high-velocity exhaust gases to obtain thrust, as in the
turbojet, a turbine rotor converts the energy of the expanding gases
to rotational shaft power. A propeller or helicopter transmission
can be connected to the engine through reduction gearing. This
energy may be extracted by the same turbine rotor that drives the
compressor, or it may be a free-power turbine which is independent of
the compressor turbine and only linked to it by the expanding gases.
The free-power turbine is the type used in Army aircraft to
harness the energy of the gases and convert this energy to rotational
shaft power. This feature of having a free-power turbine enables thepower output shaft to turn at a constant speed while the power
producing capability of the engine can be varied to accommodate the
increased loads applied to the power output shaft. Turbine engines
may be further divided into three general groups, centrifugal-flow,
axial-flow, and axial-centrifugal-flow, depending upon the type of
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1.12. ADVANTAGES OF TURBINE ENGINES
Keeping in mind the basic theory of turbine engines, compare
the advantages and disadvantages of the turbine engine with the
piston or reciprocating engine. The advantages are covered in the
subparagraphs below, and disadvantages are discussed in paragraph
1.13.
a. Power-to-weight ratio. Turbine engines have a higher
power-to-weight ratio than reciprocating engines. An example of this
is the T55-L-l11. It weighs approximately 650 pounds and delivers 3,
750 shaft horsepower. The power-to-weight ratio for this engine is
5.60 shp per pound, where the average reciprocating engine has a
power-to-weight ratio of approximately .67 shp per pound.
b. Less maintenance. Maintenance per hour of operation is
especially important in military operations. Turbine engines require
less maintenance per flying hour than reciprocating engines generally
do. As an aircraft maintenance officer, this advantage will appealto you because of a greater aircraft availability and lower
maintenance hour to flying hour ratio. The turbine engine also has
fewer moving parts than a reciprocating engine; this is also an
advantage over the reciprocating engine.
c. Less drag. Because of the design, the turbine engine has
a smaller frontal area than the reciprocating engine. A
reciprocating engine requires a large frontal area which causes a
great deal of drag on the aircraft. Turbine engines are more
streamlined in design, causing less drag. Figure 1.6 shows one of
the two nacelles that contain reciprocating engines in the old CH-37cargo helicopter. Figure 1.7 shows the smaller frontal area of the
turbine engines that power the CH-47 Chinook helicopter. Because of
this, the engine nacelles are more streamlined in design, causing
less drag.
d. Cold weather starting. The turbine engine does not
require any oil dilution or preheating of the engine before starting.
Also, once started, the reciprocating engine takes a long time to
warm up to operating temperatures, whereas the turbine engine starts
readily and is up to operating temperature immediately.
e. Low oil consumption. The turbine engine, in general, has
a lower rate of oil consumption than the reciprocating engine. The
turbine engine does not require the oil reservoir capacity to be as
large as the reciprocating engine's; because of this, a weight and
economy factor is an additional advantage.
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Figure 1.6. Reciprocating Engine Nacelles on CH-37.
Figure 1.7. Turbine Engine Nacelles on CH-47.
1.13. DISADVANTAGES OF TURBINE ENGINES
Just like everything else, along with the advantages or the
good, we have to take the disadvantages or the bad. This also holds
true with the turbine engine. The disadvantages of the turbine
engine are discussed in the following subparagraphs.
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operating theory explained in this course. Directional references
are shown in figure 1.9. Table I shows engine symbols and
abbreviations commonly used.
Figure 1.9. Directional References.
a. Directional references. Front or forward -- cold end of
engine. Rear or aft -- hot end of engine. Right and left --determined by viewing the engine from the rear. Bottom -- determined
by the location of the combustor drain valve. Top -- directly
opposite, or 180 degrees from the combustor drain valve.
These directional references hold true for most gas
turbine engines. On some the power shaft is at the end where the
exhaust gas is expelled. An engine of this design is the T73
installed on the CH-54 flying crane.
b. Engine station notation. The engine is divided into
stations to designate temperature (T) or pressure (P) measuring
locations. Figure 1.10 shows a T53-L-13, labeling the engine
stations. Any time a number is placed after the letter T or P, it
denotes a specific location in the engine.
Example: The symbol T3, denotes the relative temperature at
a specific location on the engine.
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Table I. Commonly Used Gas Turbine Engine Symbols and Abbreviations
c. Engine speed notation. The rotational speed of the engine
is represented by the capital letter N. The first rotating mass, the
gas producer has the symbol N1. Any time a number is placed after the
letter N it denotes a specific system on the gas turbine engine.
1.17. ENGINE MODEL DESIGNATIONS
Letter designators are used to differentiate the jet-
propulsion engines from reciprocating engines. A letter-number
combination identifies each type of the various gas turbine engines.
The J series designates true turbojet engines; the T designates
turboprop or turboshaft engines; and the TF, turbofan engines.
One of these letters or letter combinations begins each engine
model number, each part of which has a special significance. For
example, in the engine model number T53-L13, the T means turboshaft;
the 53 is simply the number assigned to this model by the Air Forcewhen the engine was accepted or used experimentally. The Air
Force/Army designation numbers are odd, while engines developed
originally for the Navy get even numbers. The letter L is added by
the manufacturer (in this case the Lycoming Division of
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Figure 1.10. Engine Stations.
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1.19. COMPRESSOR SECTION
The compressor is the section of the engine that produces an
increase in air pressure. It is made up of rotating and stationary
vane assemblies. The first stage compressor rotor blades accelerate
the air rearward into the first stage vane assemblies. The first
stage vane assemblies slow the air down and direct it into the second
stage compressor rotor blades. The second stage compressor rotorblades accelerate the air rearward into the second stage vane
assemblies, and so on through the compressor rotor blades and vanes
until air enters the diffuser section. The highest total air
velocity is at the inlet of the diffuser. As the air passes rearward
through the diffuser, the velocity of the air decreases and the
static pressure increases. The highest static pressure is at the
diffuser outlet.
The compressor rotor may be thought of as an air pump. The
volume of air pumped by the compressor rotor is basically
proportional to the rotor rpm. However, air density, the weight of agiven volume of air, also varies this proportional relationship. The
weight per unit volume of air is affected by temperature, compressor
air inlet pressure, humidity, and ram air pressure*. If compressor
air inlet temperature is increased, air density is reduced. If
compressor air inlet pressure is increased, air density is increased.
If humidity increases, air density is decreased. Humidity, by
comparison with temperature, and pressure changes, has a very small
effect on density. With increased forward speed, ram air pressure
increases and air temperature and pressure increase.
The following is an example of how air density affectscompressor efficiency of the T62 gas turbine. At 100 percent N1 rpm,
the compressor rotor pumps approximately 40.9 cubic feet of air per
second. At standard day static sea level conditions, 59 F outside
air temperature and 29.92" Hg barometric pressure, with 0 percent
relative humidity and 0 ram air, air density is .07651 pound per
cubic foot. Under these conditions, 40.9 cubic feet per second times
.07651 pound per cubic feet equals approximately 3.13 pounds per
second air flow through the engine. If the air density at the
compressor inlet is less than on a standard day, the weight of air
flow per second through the engine is less than 3.13 pounds per
second. If N1 is less than 100 percent rpm on a standard day, theweight of air flow per second through the engine will be less than 3.
13 due to decreased volume flow at lower rpm. Because of this, N1 rpm
varies
________________
*ram air pressure - free stream air pressure provided by the forward
motion of the engine.
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with the power output. If output power is increased, N1 rpm will
increase and vice versa. Thus, the weight of air pumped by the
compressor rotor is determined by rpm and air density.
Compressor efficiency determines the power necessary to create
the pressure rise of a given airflow, and it affects the temperature
change which takes place in the combustion chamber. Therefore, the
compressor is one of the most important components of the gas turbineengine because its efficient operation is the key to overall engine
performance. The following subparagraphs discuss the three basic
compressors used in gas turbine engines: the centrifugal-flow, the
axial-flow, and axial-centrifugal-flow compressors. The axial-
centrifugal-flow compressor is a combination of the other two and
operates with characteristics of both.
a. Centrifugal-flow compressor. Figure 1.12 shows the basic
components of a centrifugal-flow compressor: rotor, stator, and
compressor manifold.
Figure 1.12. Typical Single-stage
Centrifugal Compres-
sor
As the impeller (rotor)
revolves at high speed, air is
drawn into the blades near the
center. Centrifugal force
accelerates this air and causes
it to move outward from the axis
of rotation toward the rim of the
rotor where it is forced through
the diffuser section at high
velocity and high kinetic energy.
The pressure rise is produced byreducing the velocity of the air
in the diffuser, thereby
converting velocity energy to
pressure energy. The centrifugal
compressor is capable of a
relatively high compression ratio
per stage. This compressor is
not used on larger engines
because of size and weight.
Because of the high tip speed problem in this design, thecentrifugal compressor finds its greatest use on the smaller engines
where simplicity, flexibility of operation, and ruggedness are the
principal requirements rather than small frontal area and ability to
handle high airflows and pressures with low loss of efficiency.
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Figure 1.15. Axial-Centrifugal-Flow Compressor.
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is known as stall. All pilots are familiar with this condition and
its consequences as it applies to the wing of an aircraft. The stall
that takes place on the fixed or rotating blades of a compressor is
the same as the stalling phenomenon of an aircraft wing.
1.20. COMPRESSOR CONSTRUCTION
Centrifugal-flow compressors are usually made of titanium.The diffuser is generally manufactured of a stainless steel alloy. A
close fit is important between the compressor and its case to obtain
maximum compressor efficiency. Correct rotor assembly balancing is
essential for safe operation because of the high rpm. Balancing the
rotor can be accomplished by removing metal from specified areas of
the compressor or by using balancing weights installed in holes in
the hub of the compressor. On some engines where the compressor and
turbine wheel are balanced as a unit, special bolts and nuts having
slight variations in weight are used.
Axial-flow compressors are constructed of many differentmaterials, depending upon the load and temperature under which the
unit must operate. The rotor blades are generally cast of stainless-
steel alloy. Some manufacturers use mdybdenum coated titanium blades
to dampen vibrations on some stages of rotor blades. The clearance
between the rotor blades and the outer case is most important. Some
companies coat the inner surface of the compressor case with a soft
material that can be worn away by the blades as they expand because
of the heat generated from compressing the air. This type of
compressor uses the "wear-fit" method to form its own clearance
between the compressor case and the rotor blade tip.
Methods of attaching the blade to the disk or hub vary between
manufacturers, with the majority using some variation of the dove-
tail method to hold the rotor blades to the disk. Various other
methods are used to anchor the blades in place. Some blades do not
have a tight fit in the disk, but rather are seated by centrifugal
force during engine operation. By allowing the blades to move,
vibrational stress is reduced during start and shutdown. Stator
vanes, shown in figure 1.16, can be either solid or hollow
construction, and are connected together at their tips by a shroud.
This shrouding serves two purposes. First, it provides support, and
second, it provides the necessary air seal between rotating andstationary parts. Most manufacturers use the split compressor cases,
while some others favor a weldment, forming a continuous case. The
advantages of the split case lie in the fact that the compressor and
stator blades are readily available to inspection. The one-piece
case offers simplicity and strength because it is one piece; in most
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Figure 1.17. One-Piece Compressor Case.
1.21. COMBUSTION SECTION
Today, three basic combustion chambers are in use. They are
the annular combustion chamber, the can type, and the combination of
the two called the can-annular. Variations of these basic systems
are used in a number of engines. The three systems are discussed
individually in the following subparagraphs. The most commonly used
gas turbine engine in Army aircraft is the annular reverse-Row type.
The combustion section contains the combustion chambers, igniter
plugs, and fuel nozzles or vaporizing tubes. It is designed to burn
a fuel-air mixture and deliver the combusted gases to the turbine at
a temperature which will not exceed the allowable limit at the
turbine inlet.
Fuel is introduced at the front end of the burner in a highly
atomized spray from the fuel nozzles. Combustion air flows in around
the fuel nozzle and mixes with the fuel to form a correct fuel-air
mixture. This is called primary air and represents approximately 25
percent of total air taken into the engine. The fuel-air mixture
which is to be burned is a ratio of 15 parts of air to 1 part of fuel
by weight. The remaining 75 percent of the air is used to form an
air blanket around the burning gases and to lower the temperature.
This temperature may reach as high as 3500 F. By using 75 percent of
the air for cooling, the temperature operating range can be brought
down to about half, so the turbine section will not be destroyed by
excessive heat. The air used for burning is
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called primary air- and that for cooling is secondary air. The
secondary air is controlled and directed by holes and louvers in the
combustion chamber liner.
Igniter plugs function only during starting, being cut out of
the circuit as soon as combustion is self-supporting. On engine
shutdown, or, if the engine fails to start, the combustion chamber
drain valve, a pressure-actuated valve, automatically drains anyremaining unburned fuel from the combustion chamber. All combustion
chambers contain the same basic elements: a casing or outer shell, a
perforated inner liner or flame tube, fuel nozzles, and some means of
initial ignition. The combustion chamber must be of light
construction and is designed to burn fuel completely in a high
velocity airstream. The combustion chamber liner is an extremely
critical engine part because of the high temperatures of the flame.
The liner is usually constructed of welded high-nickel steel. The
most severe operating periods in combustion chambers are encountered
in the engine idling and maximum rpm ranges. Sustained operation
under these conditions must be avoided to prevent combustion chamberliner failure.
a. The annular-type combustion chamber shown in figure 1.18
is used in engines of the axial-centrifugal-flow compressor de-
1. ANNULAR TYPE COMBUSTION CHAMBER LINER
2. COMBUSTION CHAMBER HOUSING ASSEMBLY
Figure 1.18. Annular-type Combustion Chamber.
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Ignition is accomplished during the starting cycle. The
igniter plug is located in the combustion liner adjacent to the start
fuel nozzle. The Army can-type engine employs a single can-type
combustor.
c. Can-annular combustion chamber. This combustion chamber
uses characteristics of both annular and can-type combustion
chambers. The can-annular combustion chamber consists of an outershell, with a number of individual cylindrical liners mounted about
the engine axis as shown in figure 1.20. The combustion chambers are
completely surrounded by the airflow that enters the liners through
various holes and louvers. This air is mixed with fuel which has
been sprayed under pressure from the fuel nozzles. The fuel-air
mixture is ignited by igniter plugs, and the flame is then carried
through the crossover tubes to the remaining liners. The inner
casing assembly is both a support and a heat shield; also, oil lines
run through it.
Figure 1.20. Can-Annular Combustion Chamber.
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1.22. TURBINE SECTION
A portion of the kinetic energy of the expanding gases is
extracted by the turbine section, and this energy is transformed into
shaft horsepower which is used to drive the compressor and
accessories. In turboprop and turboshaft engines, additional turbine
rotors are designed to extract all of the energy possible from the
remaining gases to drive a powershaft.
a. Types of turbines. Gas turbine manufacturers have
concentrated on the axial-flow turbine shown in figure 1.21. This
turbine is used in all gas-turbine-powered aircraft in the Army
today. However, some manufacturers are building engines with a
radial inflow turbine, illustrated in figure 1.22. The radial inflow
turbine
Figure 1.21. Axial-flow Turbine Rotor.
Figure 1.22. Radial Inflow
Turbine.
has the advantage of ruggedness
and simplicity, and it is
relatively inexpensive and easy
to manufacture when compared to
the axial-flow turbine. The
radial flow turbine is similar in
design and construction to the
centrifugal-flow compressordescribed in paragraph 1.19a.
Radial turbine wheels used for
small
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engines are well suited for a higher range of specific speeds and
work at relatively high efficiency.
The axial-flow turbine consists of two main elements, a set of
stationary vanes followed by a turbine rotor. Axial-flow turbines
may be of the single-rotor or multiple-rotor type. A stage consists
of two main components: a turbine nozzle and a turbine rotor or
wheel, as shown in figure 1.21. Turbine blades are of two basictypes, the impulse and the reaction. Modern aircraft gas turbines
use blades that have both impulse and reaction sections, as shown in
figure 1.23.
Figure 1.23. Impulse-Reaction Turbine Blade.
The stationary part of the turbine assembly consists of a row
of contoured vanes set at a predetermined angle to form a series of
small nozzles which direct the gases onto the blades of the turbinerotor. For this reason, the stationary vane assembly is usually
cal